- Email: [email protected]
The effect of acidity and pore structure of catalysts on the dehydroisomerization of n-butane to isobutene
Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.
2359
THE EFFECT OF ACIDITY AND PORE S T R U C T U R E OF C A T A L Y S T S ON THE D E H Y D R O I S O M E R I Z A T I O N OF n-BUTANE TO I S O B U T E N E Wei, Y., Wang, G., Liu, Z.*, Xu, L. and Xie, P. Natural Gas Utilization and Applied Catalysis Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, PO Box 110, Dalian 116023, China. *E-mail: [email protected]
ABSTRACT Four molecular sieves of A1PO-11 and SAPO-5, 11, 34 were synthesized and characterized. The results showed the differences of acidity and porous structure. The four samples were modified by Pd and used in the direct transformation of n-butane to isobutene. Pd/A1PO-11, Pd/SAPO-5 and Pd/SAPO-34 showed low catalytic activity and poor selectivity for isobutene, while Pd/SAPO-11 showed relatively high activity and high isobutene selectivity. The effect of acidity and porous structure on the support Pd and the catalytic performance were discussed. Medium strong acidity and suitable pore geometry were necessary in this reaction.
INTRODUCTION Aluminophophates(A1POs) and silicoaluminophosphates (SAPOs) molecular sieves form another class of microporous crystalline materials [1] comparable to the well-known zeolites. Some SAPOs have been reported as useful catalysts for a variety of chemical reactions [2-6]. The incorporation of Si into the A1PO framework is the key to the formation of acid sites, and the change of Si environment in the structure would cause the variation of acid-catalytic properties [7-10]. Among SAPO molecular sieves, SAPO-5, l l, 34, which possesses AFI, AEL and CHA toplogy respectively, have been proved to be very active and selective catalysts for many reactions [14-18]. Different processes for isobutene production including skeletal isomerization of n-butene received much attention recently with the increasing demand of isobutene in industry. Considering the abundant supply of n-butane from natural gas and refinery streams, n-butane is the preferred raw material for isobutene production. Compared to the two-step production of isobutene from n-butane comprising isomerization and dehydrogenation units, direct transformation of n-butane to isobutene is an interesting novel process [ 19-24], which maybe realized over the bifunctional catalysts combining the dehydrogenation and the isomerization functions. In the present work, the catalysts of Pd modified A1PO and SAPOs were prepared and used in dehydroisomerization of n-butane to isobutene. The effects of acidity and porous structure of the molecular sieve supports on the catalytic performance were investigated in detail.
EXPERIMENTAL Synthesis of molecular sieve A1PO-11, SAPO-34, SAPO-11 and SAPO-5 were synthesized by hydrothermal method. Pseudoboehmite, orthophosphoric acid (85 wt %) and colloidal silica were used as the sources of aluminum, phosphorus and silicon respectively. Di(n-propyl)amine was used as the template for A1PO-11 and SAPO-11and triethylamine was used as the template for SAPO-5 and SAPO-34. To prepare the starting gel, the pseudoboehmite was mixed with orthophosphoric acid and distilled water and stirred at room temperature for 2 h. To this mixture, template and colloidal silica were subsequently added under stirring and then stirred continuously for 2 h. The chemical composition of the starting gel obtained was lishted in Table 1. The gels were sealed in the stainless-steel autoclave lined with polytetrafluoroethylene (PTFE) and heated, following the procedure reported in the literature [1]. The solid products were filtrated, washed, and dried at 373 K for 3 h, then calcined at 823 K for 6 h to completely remove the template.
2360 Table 1. Molar composition of the starting gel for molecular sieve synthesis. Sample SAPO-34 A1PO- 11 SAPO- 11 SAPO-5
Molar composition of the starting gel mixtures 3.0Et3NH: 1.0P205:1.0A1203:0.2SIO2:50H20 1.0nPr2NH: 1.0P205:1.0A1203:40H20 1.0nPr2NH: 1.0P205:1.0A1203:0.4SIO2:55H20 1.2Et3NH: 1.0P205:1.0A1203:0.4SIO2:50H20
Catalyst preparation The calcined A1PO-11, SAPO-5, SAPO-11 and SAPO-34 powder were pressed to tablets without any binder, crushed and sieved to 0.5-1.0 mm particles for catalyst preparation. The wetness impregnation method was used. The molecular sieves were degassed under vacuum for 1 h at room temperature, and then impregnated with an aqueous solution containing the desired amount of precursor Pd(NH3)4C12. The metal loading was 0.3 wt% Pd for each catalyst sample. After impregnation, the samples were dried at 373 K for 10 h and calcined at 773 K in air for 2 h. These catalysts are denoted Pd/AIPO-11, Pd/SAPO-5, Pd/SAPO-11 and Pd/SAPO-34.
Characterization Crystallinity and phase purity of the as-synthesized samples were characterized by powder X-ray diffraction (XRD, RIGAKU D/max-rb powder diffractometer) with Cu Kc~ radiation. The chemical composition of the samples was determined with X-ray fluorescence technique (Bruker SRS-3400 XRF spectrometer). Surface area and porosity measurements were carried out by means of nitrogen adsorption at liquid nitrogen temperature with Micrometric 2010. The surface area was calculated according to the BET isothermal equation, and the microporous volume was evaluated by the Horvath-Kawazoe method. Acidity of the samples was characterized using temperature programmed desorption (TPD) spectra of ammonia with a Micrometric 2910 chemisorption meter. The sample of 200 mg was heated up to 873 K and maintained at this temperature for 1 h in He stream (20 ml/min). The samples were then cooled down to 373 K in a flow of He and exposed to NH3, which were injected into the He stream (40ml/min). Desorption of NH3 was monitored at a temperature ramp from 373 K to 873 K.
Catalytic testing The catalytic tests were performed using a fixed bed reactor system at atmosphere pressure. 0.5 g of catalyst was loaded into a stainless steel reactor tube with an inner diameter of 5 mm. The catalyst bed had a length of about 50 mm and was supported by quartz wool on both sides. The reaction temperature was 573 K-623 K. Before reaction, the catalysts were reduced in situ with H2 (60 cm3/min) at 773 K for 1 h, then switch from H2 to the feed, a mixture of H2 and n-butane (the molar ratio of H2/n-butane was 2). The weight hourly space velocity (WHSV) was 1.98 h-1 for n-butane. The reaction products were analyzed on-line by a Varian GC3800 gas chromatograph equipped with a FID detector and an 18 m • 0.46mm A1203 capillary column. R E S U L T S AND D I S C U S S I O N The XRD patterns of the as-synthesized molecular sieves are shown in Fig. 1. The position and the intensity of the diffraction peaks are identical to those reported in the literature [1 ]. High intensity of XRD lines and absence of any baseline drift indicate that they are highly crystalline with no obvious impurity phase. The chemical composition, BET and microporous volume of the synthesized samples are listed in Table 2 for comparing. With modulating the composition of the starting gel, three SAPOs with nearly the same chemical composition of the crystalline products were obtained. Among the four samples, SAPO-34 possessed the highest specific area and microporous volume.
2361
5,00
i0:00
20:~)
30;CKJ
4(~ (!i~
5(),,(,)(,i~
Figure 1. XRD patterns of the crystalline samples (1) SAPO-5 (2) A1PO-11 (3) SAPO-11 (4) SAPO-34. Table 2. Molar compositions, specific area and microporous volume of the crystalline products. Product SAPO-34 A1PO-11 SAPO-11 SAPO-5
Mole composition A10.50P0.44Si0.0602 A10.50P0.5002 A10.50P0.44Si0.0602 A10.50P0.44Si0.0602
Specific area (m2/g)
Microporous volume (cm3/g)
554 120 137 158
0.273 0.063 0.067 0.063
Pore geometry difference exists between SAPO-5, 11 and 34 molecular sieves, which are the most important members of SAPOs and have been studied intensively in the last two decades. The molecular sieve pore geometry is of key importance for the catalytic activity. In our study, these three types of SAPOs were synthesized and used for the dehydroisomerization reaction of n-butane. The reaction results of Pd modified SAPO-5, SAPO-11 and SAPO-34 in Fig. 2 showed the effect of pore geometry on the catalytic properties. For Pd/SAPO-11, n-butane was mainly transferred to isobutene and other butenes. A small quantity (<20%) of cracking and hydrogenolysis products was also obtained. But for Pd/SAPO-34 and Pd/SAPO-5, an abrupt change in product distribution and catalytic activity was observed. Though the dehydrogenation product was also obtained, the simultaneous skeletal isomerization is virtually suppressed over the two samples. The pores of the catalyst are thought to accommodate the intermediate of isomerization and the metal particle with dehydrogenation function, so pore geometry of the molecular sieve may play an important role for the catalytic performance. Among the catalysts, SAPO-34 has the largest micropore volume and area, but its 8-member ring pore opening may sterically inhibit the isomerization of n-butane or n-butene; for Pd/SAPO-5, with the 12-ring pore-opening molecular sieve, no sterical inhibition for isomerization exists in this catalyst, the low isomerization selectivity may come from its weak acidity. The acidic center from the acidic support is considered to be necessary in the framework isomerization. For Pd/A1PO-11 and Pd/SAPO-11, the molecular sieve as the support possessing the same AEL topology, the difference of the catalytic performance should be attributed to the acidity. A1PO-11 showed the weakest acidity (see Fig. 3), which results from the absence of acidic sites over the neutral framework, so over the catalyst of Pd/A1PO-11, the dehydrogenation catalyzed by noble metal is prominent; while over the Pd/SAPO-11, with the medium-acidic support, the dehydrogenation and isomerization can happen simultaneously.
2362
~3
~.13
:=-~,
~.~ 43.6
41,~
iiii!!iii
8E~ i iiiiii!i6 , 2 3
Pd/SA PO-34 m Convers ion
m ~
I I cl'~ii~iii I ..........~,i~i~ ............. II'~"~i~i~
13~8531~L~ I ~i
I
Pd/AI PO-11 CI-C3
~ ,132 12.7 iii'~:~i:i;iiO. iii!i:~,iii':i
I
i:i!: : ~ii;?: ~,licii~
Pd/SAPO-11
Pd/SA PO-5
~!!!isobutene
i butenes
Figure 2. The catalyticperformance of Pd/AIPO-l I and Pd/SAPOs. 475
.......
_ .......... ......... 873
773
673
' 12 573
473
373
Temperature (K) Figure 3. NH3-TPD patterns ofAIPO-I l and SAPOs (l) SAPO-5 (2) AIPO-I l (3) SAPO-I l (4) SAPO-34. Si incorporation generates acidic sites in AIPO framework. NH3-TPD was employed in this study to test the acidity of the synthesized molecular sieves. Comparing the NH3-desorption peaks in the Fig 3, it is easy to find the acidity difference among three SAPO with different structure and same chemical composition. The acid strength roughly followed the order that: SAPO-34>SAPO-I I>SAPO-5; while we know the order of pore opening is: SAPO-34
2363 support showed high dehydrogenation selectivity, following the dehydrogenation selectivity order of Pd/A1PO-11 > Pd/SAPO-5 > Pd/SAPO-11 >Pd/SAPO-34. This butenes selectivity is in a reverse order to the order of acidity. CONCLUSION A1PO-11, SAPO-5, SAPO-11 and SAPO-34 were synthesized. XRD indicated the different crystal structures for these samples. NH3-TPD and physical adsorption of nitrogen tested the difference of the four samples in acidity and pore distribution of these samples. The acid strength order is: SAPO-34 >SAPO-11 >SAPO-5 >AIPO-11. The Pd modified catalysts based on these molecular sieves were used for the dehydroisomerization of n-butane to isobutene. Poor activity and selectivity to isobutene were obtained over the catalyst based on the SAPO-5 or A1PO-11 with weak acidity and SAPO-34 with narrow pore opening. The Pd modified SAPO-11 with 10-member ring pore opening showed highest activity and selectivity to isobutene among the four catalysts. The characterization data and reaction result show the effect of acidity and pore structure of the catalysts on the catalytic properties. A medium strong acidity and suitable pore geometry maybe needed in this transformation of n-butane to isobutene in one step. REFERENCES 1. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. Flanigen, US Patent 4,440,871 (1984). 2. I.M. Dahl, S. Kolboe, Catal. Lett. 20 (1993) 329. 3. I.M. Dahl, S. Kolboe, J. Catal. 149 (1994) 458. 4. I.M. Dahl, S. Kolboe, J. Catal. 161 (1996) 304. 5. J.M. Campelo, F. Lafont, J.M. Marinas, J. Chem. Soc., Faraday Trans. 91 (10) (1995) 1551. 6. J.M. Campelo, F. Lafont, J. M. Marinas, Appl. Catal. 152 (1997) 5. 7. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. Flanigen, J. Am. Chem. Soc. 106 (1984) 6092. 8. R.B. Borade, A. Clearfield, J. Mol. Catal. 88 (1994) 249. 9. D.B. Akolekar, J. Catal. 149 (1994) 1. 10. R. Ramachandra Rao, S.J. Kulkarni, M. Subrahmanyam, A.V. Rama Rao, Zeolites 16 (1996) 254. 11. G. J. Gajda, US Patent 5,132,484 (1992). L.H. Gielgens, I.H.E. Veenstra, V. Ponec, M.J. Haanepen, J.H.C. Van Hooff, Catal. Lett. 32 (1995) 195. 12. S.M. Yang, D.H. Guo, J.S. Lin, G.T. Wang, Stud. Surf. Sci. Catal. 84 (1994) 1677. 13. S. J. Miller, US Patent 5,135,638 (1992). 14. J.M. Campels, F. Lafont, J.M. Marinas, J. Catal. 156 (1995) 11. 15. R.J. Taylor, R.H. Detty, Appl. Catal., 119 (1994) 121. 16. P. Meriaudeau, V.A. Tuan, V.T. Nghiem, S.Y. Lai, L.N. Hung, C. Naccache, J. Catal. 169 (1997) 55. 17. P. Meriaudeau, V.A. Tuan, F. Lefebvre, V.T. Nghiem, C. Naccache, Micropor. Mesopor. Mater. 22 (1998) 435. 18. D.L. Sikkenga, T.D. Nevitt, N.F. Jerome, US Patent 4,433,190 (1984). 19. C.L. O'Young, J.E. Browne, J.F. Matteo, R.A. Sawicki, J. Hazen, US Patent 5,198,597 (1993). 20. R. Byggningsbacka, N. Kumar, L.-E. Lindfors, Catal. Lett. 55 (1998) 173. 21. B. Didillion, C. Travers and J-P Burzynski, US Patent 5,866,746 (1999). 22. A. Vieira, M.A. Tovar, C. Pfaff, P. Betancourt, B. Mendez, C.M. Lopez, F.J. Machado, J. Goldwasser, M.M. Ramirez de Agudelo, M. Houalla, J. Mol. Catal. A:, 144 (1999) 101. 23. G.D. Pirngruber, K. Seshan, J.A. Lercher, J. Catal. 186 (1999) 188.
2359
THE EFFECT OF ACIDITY AND PORE S T R U C T U R E OF C A T A L Y S T S ON THE D E H Y D R O I S O M E R I Z A T I O N OF n-BUTANE TO I S O B U T E N E Wei, Y., Wang, G., Liu, Z.*, Xu, L. and Xie, P. Natural Gas Utilization and Applied Catalysis Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, PO Box 110, Dalian 116023, China. *E-mail: [email protected]
ABSTRACT Four molecular sieves of A1PO-11 and SAPO-5, 11, 34 were synthesized and characterized. The results showed the differences of acidity and porous structure. The four samples were modified by Pd and used in the direct transformation of n-butane to isobutene. Pd/A1PO-11, Pd/SAPO-5 and Pd/SAPO-34 showed low catalytic activity and poor selectivity for isobutene, while Pd/SAPO-11 showed relatively high activity and high isobutene selectivity. The effect of acidity and porous structure on the support Pd and the catalytic performance were discussed. Medium strong acidity and suitable pore geometry were necessary in this reaction.
INTRODUCTION Aluminophophates(A1POs) and silicoaluminophosphates (SAPOs) molecular sieves form another class of microporous crystalline materials [1] comparable to the well-known zeolites. Some SAPOs have been reported as useful catalysts for a variety of chemical reactions [2-6]. The incorporation of Si into the A1PO framework is the key to the formation of acid sites, and the change of Si environment in the structure would cause the variation of acid-catalytic properties [7-10]. Among SAPO molecular sieves, SAPO-5, l l, 34, which possesses AFI, AEL and CHA toplogy respectively, have been proved to be very active and selective catalysts for many reactions [14-18]. Different processes for isobutene production including skeletal isomerization of n-butene received much attention recently with the increasing demand of isobutene in industry. Considering the abundant supply of n-butane from natural gas and refinery streams, n-butane is the preferred raw material for isobutene production. Compared to the two-step production of isobutene from n-butane comprising isomerization and dehydrogenation units, direct transformation of n-butane to isobutene is an interesting novel process [ 19-24], which maybe realized over the bifunctional catalysts combining the dehydrogenation and the isomerization functions. In the present work, the catalysts of Pd modified A1PO and SAPOs were prepared and used in dehydroisomerization of n-butane to isobutene. The effects of acidity and porous structure of the molecular sieve supports on the catalytic performance were investigated in detail.
EXPERIMENTAL Synthesis of molecular sieve A1PO-11, SAPO-34, SAPO-11 and SAPO-5 were synthesized by hydrothermal method. Pseudoboehmite, orthophosphoric acid (85 wt %) and colloidal silica were used as the sources of aluminum, phosphorus and silicon respectively. Di(n-propyl)amine was used as the template for A1PO-11 and SAPO-11and triethylamine was used as the template for SAPO-5 and SAPO-34. To prepare the starting gel, the pseudoboehmite was mixed with orthophosphoric acid and distilled water and stirred at room temperature for 2 h. To this mixture, template and colloidal silica were subsequently added under stirring and then stirred continuously for 2 h. The chemical composition of the starting gel obtained was lishted in Table 1. The gels were sealed in the stainless-steel autoclave lined with polytetrafluoroethylene (PTFE) and heated, following the procedure reported in the literature [1]. The solid products were filtrated, washed, and dried at 373 K for 3 h, then calcined at 823 K for 6 h to completely remove the template.
2360 Table 1. Molar composition of the starting gel for molecular sieve synthesis. Sample SAPO-34 A1PO- 11 SAPO- 11 SAPO-5
Molar composition of the starting gel mixtures 3.0Et3NH: 1.0P205:1.0A1203:0.2SIO2:50H20 1.0nPr2NH: 1.0P205:1.0A1203:40H20 1.0nPr2NH: 1.0P205:1.0A1203:0.4SIO2:55H20 1.2Et3NH: 1.0P205:1.0A1203:0.4SIO2:50H20
Catalyst preparation The calcined A1PO-11, SAPO-5, SAPO-11 and SAPO-34 powder were pressed to tablets without any binder, crushed and sieved to 0.5-1.0 mm particles for catalyst preparation. The wetness impregnation method was used. The molecular sieves were degassed under vacuum for 1 h at room temperature, and then impregnated with an aqueous solution containing the desired amount of precursor Pd(NH3)4C12. The metal loading was 0.3 wt% Pd for each catalyst sample. After impregnation, the samples were dried at 373 K for 10 h and calcined at 773 K in air for 2 h. These catalysts are denoted Pd/AIPO-11, Pd/SAPO-5, Pd/SAPO-11 and Pd/SAPO-34.
Characterization Crystallinity and phase purity of the as-synthesized samples were characterized by powder X-ray diffraction (XRD, RIGAKU D/max-rb powder diffractometer) with Cu Kc~ radiation. The chemical composition of the samples was determined with X-ray fluorescence technique (Bruker SRS-3400 XRF spectrometer). Surface area and porosity measurements were carried out by means of nitrogen adsorption at liquid nitrogen temperature with Micrometric 2010. The surface area was calculated according to the BET isothermal equation, and the microporous volume was evaluated by the Horvath-Kawazoe method. Acidity of the samples was characterized using temperature programmed desorption (TPD) spectra of ammonia with a Micrometric 2910 chemisorption meter. The sample of 200 mg was heated up to 873 K and maintained at this temperature for 1 h in He stream (20 ml/min). The samples were then cooled down to 373 K in a flow of He and exposed to NH3, which were injected into the He stream (40ml/min). Desorption of NH3 was monitored at a temperature ramp from 373 K to 873 K.
Catalytic testing The catalytic tests were performed using a fixed bed reactor system at atmosphere pressure. 0.5 g of catalyst was loaded into a stainless steel reactor tube with an inner diameter of 5 mm. The catalyst bed had a length of about 50 mm and was supported by quartz wool on both sides. The reaction temperature was 573 K-623 K. Before reaction, the catalysts were reduced in situ with H2 (60 cm3/min) at 773 K for 1 h, then switch from H2 to the feed, a mixture of H2 and n-butane (the molar ratio of H2/n-butane was 2). The weight hourly space velocity (WHSV) was 1.98 h-1 for n-butane. The reaction products were analyzed on-line by a Varian GC3800 gas chromatograph equipped with a FID detector and an 18 m • 0.46mm A1203 capillary column. R E S U L T S AND D I S C U S S I O N The XRD patterns of the as-synthesized molecular sieves are shown in Fig. 1. The position and the intensity of the diffraction peaks are identical to those reported in the literature [1 ]. High intensity of XRD lines and absence of any baseline drift indicate that they are highly crystalline with no obvious impurity phase. The chemical composition, BET and microporous volume of the synthesized samples are listed in Table 2 for comparing. With modulating the composition of the starting gel, three SAPOs with nearly the same chemical composition of the crystalline products were obtained. Among the four samples, SAPO-34 possessed the highest specific area and microporous volume.
2361
5,00
i0:00
20:~)
30;CKJ
4(~ (!i~
5(),,(,)(,i~
Figure 1. XRD patterns of the crystalline samples (1) SAPO-5 (2) A1PO-11 (3) SAPO-11 (4) SAPO-34. Table 2. Molar compositions, specific area and microporous volume of the crystalline products. Product SAPO-34 A1PO-11 SAPO-11 SAPO-5
Mole composition A10.50P0.44Si0.0602 A10.50P0.5002 A10.50P0.44Si0.0602 A10.50P0.44Si0.0602
Specific area (m2/g)
Microporous volume (cm3/g)
554 120 137 158
0.273 0.063 0.067 0.063
Pore geometry difference exists between SAPO-5, 11 and 34 molecular sieves, which are the most important members of SAPOs and have been studied intensively in the last two decades. The molecular sieve pore geometry is of key importance for the catalytic activity. In our study, these three types of SAPOs were synthesized and used for the dehydroisomerization reaction of n-butane. The reaction results of Pd modified SAPO-5, SAPO-11 and SAPO-34 in Fig. 2 showed the effect of pore geometry on the catalytic properties. For Pd/SAPO-11, n-butane was mainly transferred to isobutene and other butenes. A small quantity (<20%) of cracking and hydrogenolysis products was also obtained. But for Pd/SAPO-34 and Pd/SAPO-5, an abrupt change in product distribution and catalytic activity was observed. Though the dehydrogenation product was also obtained, the simultaneous skeletal isomerization is virtually suppressed over the two samples. The pores of the catalyst are thought to accommodate the intermediate of isomerization and the metal particle with dehydrogenation function, so pore geometry of the molecular sieve may play an important role for the catalytic performance. Among the catalysts, SAPO-34 has the largest micropore volume and area, but its 8-member ring pore opening may sterically inhibit the isomerization of n-butane or n-butene; for Pd/SAPO-5, with the 12-ring pore-opening molecular sieve, no sterical inhibition for isomerization exists in this catalyst, the low isomerization selectivity may come from its weak acidity. The acidic center from the acidic support is considered to be necessary in the framework isomerization. For Pd/A1PO-11 and Pd/SAPO-11, the molecular sieve as the support possessing the same AEL topology, the difference of the catalytic performance should be attributed to the acidity. A1PO-11 showed the weakest acidity (see Fig. 3), which results from the absence of acidic sites over the neutral framework, so over the catalyst of Pd/A1PO-11, the dehydrogenation catalyzed by noble metal is prominent; while over the Pd/SAPO-11, with the medium-acidic support, the dehydrogenation and isomerization can happen simultaneously.
2362
~3
~.13
:=-~,
~.~ 43.6
41,~
iiii!!iii
8E~ i iiiiii!i6 , 2 3
Pd/SA PO-34 m Convers ion
m ~
I I cl'~ii~iii I ..........~,i~i~ ............. II'~"~i~i~
13~8531~L~ I ~i
I
Pd/AI PO-11 CI-C3
~ ,132 12.7 iii'~:~i:i;iiO. iii!i:~,iii':i
I
i:i!: : ~ii;?: ~,licii~
Pd/SAPO-11
Pd/SA PO-5
~!!!isobutene
i butenes
Figure 2. The catalyticperformance of Pd/AIPO-l I and Pd/SAPOs. 475
.......
_ .......... ......... 873
773
673
' 12 573
473
373
Temperature (K) Figure 3. NH3-TPD patterns ofAIPO-I l and SAPOs (l) SAPO-5 (2) AIPO-I l (3) SAPO-I l (4) SAPO-34. Si incorporation generates acidic sites in AIPO framework. NH3-TPD was employed in this study to test the acidity of the synthesized molecular sieves. Comparing the NH3-desorption peaks in the Fig 3, it is easy to find the acidity difference among three SAPO with different structure and same chemical composition. The acid strength roughly followed the order that: SAPO-34>SAPO-I I>SAPO-5; while we know the order of pore opening is: SAPO-34
2363 support showed high dehydrogenation selectivity, following the dehydrogenation selectivity order of Pd/A1PO-11 > Pd/SAPO-5 > Pd/SAPO-11 >Pd/SAPO-34. This butenes selectivity is in a reverse order to the order of acidity. CONCLUSION A1PO-11, SAPO-5, SAPO-11 and SAPO-34 were synthesized. XRD indicated the different crystal structures for these samples. NH3-TPD and physical adsorption of nitrogen tested the difference of the four samples in acidity and pore distribution of these samples. The acid strength order is: SAPO-34 >SAPO-11 >SAPO-5 >AIPO-11. The Pd modified catalysts based on these molecular sieves were used for the dehydroisomerization of n-butane to isobutene. Poor activity and selectivity to isobutene were obtained over the catalyst based on the SAPO-5 or A1PO-11 with weak acidity and SAPO-34 with narrow pore opening. The Pd modified SAPO-11 with 10-member ring pore opening showed highest activity and selectivity to isobutene among the four catalysts. The characterization data and reaction result show the effect of acidity and pore structure of the catalysts on the catalytic properties. A medium strong acidity and suitable pore geometry maybe needed in this transformation of n-butane to isobutene in one step. REFERENCES 1. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. Flanigen, US Patent 4,440,871 (1984). 2. I.M. Dahl, S. Kolboe, Catal. Lett. 20 (1993) 329. 3. I.M. Dahl, S. Kolboe, J. Catal. 149 (1994) 458. 4. I.M. Dahl, S. Kolboe, J. Catal. 161 (1996) 304. 5. J.M. Campelo, F. Lafont, J.M. Marinas, J. Chem. Soc., Faraday Trans. 91 (10) (1995) 1551. 6. J.M. Campelo, F. Lafont, J. M. Marinas, Appl. Catal. 152 (1997) 5. 7. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. Flanigen, J. Am. Chem. Soc. 106 (1984) 6092. 8. R.B. Borade, A. Clearfield, J. Mol. Catal. 88 (1994) 249. 9. D.B. Akolekar, J. Catal. 149 (1994) 1. 10. R. Ramachandra Rao, S.J. Kulkarni, M. Subrahmanyam, A.V. Rama Rao, Zeolites 16 (1996) 254. 11. G. J. Gajda, US Patent 5,132,484 (1992). L.H. Gielgens, I.H.E. Veenstra, V. Ponec, M.J. Haanepen, J.H.C. Van Hooff, Catal. Lett. 32 (1995) 195. 12. S.M. Yang, D.H. Guo, J.S. Lin, G.T. Wang, Stud. Surf. Sci. Catal. 84 (1994) 1677. 13. S. J. Miller, US Patent 5,135,638 (1992). 14. J.M. Campels, F. Lafont, J.M. Marinas, J. Catal. 156 (1995) 11. 15. R.J. Taylor, R.H. Detty, Appl. Catal., 119 (1994) 121. 16. P. Meriaudeau, V.A. Tuan, V.T. Nghiem, S.Y. Lai, L.N. Hung, C. Naccache, J. Catal. 169 (1997) 55. 17. P. Meriaudeau, V.A. Tuan, F. Lefebvre, V.T. Nghiem, C. Naccache, Micropor. Mesopor. Mater. 22 (1998) 435. 18. D.L. Sikkenga, T.D. Nevitt, N.F. Jerome, US Patent 4,433,190 (1984). 19. C.L. O'Young, J.E. Browne, J.F. Matteo, R.A. Sawicki, J. Hazen, US Patent 5,198,597 (1993). 20. R. Byggningsbacka, N. Kumar, L.-E. Lindfors, Catal. Lett. 55 (1998) 173. 21. B. Didillion, C. Travers and J-P Burzynski, US Patent 5,866,746 (1999). 22. A. Vieira, M.A. Tovar, C. Pfaff, P. Betancourt, B. Mendez, C.M. Lopez, F.J. Machado, J. Goldwasser, M.M. Ramirez de Agudelo, M. Houalla, J. Mol. Catal. A:, 144 (1999) 101. 23. G.D. Pirngruber, K. Seshan, J.A. Lercher, J. Catal. 186 (1999) 188.